Medical Claims Payable (Narrative) (Details) $ in Millions	Sep. 30, 2018 USD ($)
Short-duration Insurance Contracts, Claim Year 2018 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2,519
Short-duration Insurance Contracts, Claim Year 2018 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	4,742
Short-duration Insurance Contracts, Claim Year 2017 [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	192
Short-duration Insurance Contracts, Claim Year 2017 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	116
Short-duration Insurance Contracts, Claim Year 2016 and Prior [Member] | Commercial Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	18
Short-duration Insurance Contracts, Claim Year 2016 and Prior [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 33